1. Field of the Invention
The present invention relates to a hot-water supply apparatus which heats water to supply hot water. In particular, it relates to the prevention of freezing of a water pipe and so on at a cold period.
2. Description of the Related Art
In a hot-water supply apparatus which makes a heat source by burning a fuel gas, an entrance water temperature is detected by providing an entrance water temperature sensor in a water pipe. When an entrance water temperature below a set value is detected, a power source is given to a heater which is installed at the water pipe, and the water pipe is heated by the heater. By this, freezing in the water pipe is prevented. Further, in a hot-water supply apparatus which is installed in the inside of a house, since cold air enters from an exhaust pipe and so on, a fall in temperature is detected by a hot-water temperature sensor, and a heater is driven. By this, the prevention of freezing is designed to be performed.
In connection with the like of anti-freezing of a hot-water supply apparatus, the Japanese Laid Open Publications No. 10-26413, No. 10-227526, No. 11-159874 and No. 2003-207207 are in existence. The publication No. 10-26413 discloses a hot water feeding device in which combustion is started when a detected value of a hot-water temperature sensor becomes a temperature below a threshold value, and the combustion is maintained until the hot-water temperature sensor detects a value above the threshold value. In the publication No. 10-227526, combustion is put ON/OFF according to a detected value of a hot-water temperature/entrance-water temperature sensor, and freezing of a water pipe which is nearer an exit than a heat exchanger is prevented by making a heater operate based on a temperature-sensitive sensor. Further, the publication No.11-159874 discloses a method which confirms the presence/absence of freezing, for example, at the time when a power source is supplied for the first time. This publication discloses that combustion for the prevention of freezing is performed if an outgoing hot-water temperature and/or an entrance water temperature as an example becomes a temperature below a threshold value, this method decides on freezing if a temperature does not rise, and the combustion is stopped after this. Furthermore, the publication No. 2003-207207 discloses a method which heats a water pipe by driving a heater and a fan if a temperature sensor detects a freezing temperature.
By the way, in the United States of America and so on where mounting a draft hood on an exhaust pipe provided to a hot-water supply apparatus is not permitted, cold air which invades an exhaust pipe by an adverse wind refrigerates a heat exchanger at a cold period. Because of this, freezing is generated in a water pipe. Even if a heater is provided to the water pipe and it is heated, it is impossible to prevent the freezing if an ambient temperature falls extremely.
Further, in a high-efficiency hot-water supply apparatus which has a secondary heat exchanger absorbing latent heat from combustion exhaust passed through a primary heat exchanger, there is no space to attach a heater because the whole of the heat exchanger is surrounded by a radiation shield. Because of this, there is a method which prevents freezing by sending indoor air by means of a fan motor. However, even if the indoor air is sent, keeping warmth of the side of the secondary heat exchanger becomes insufficient because heat is absorbed by a side of the primary heat exchanger. Hence, it is feared that freezing of the side of the secondary heat exchanger can not be prevented.
The problems mentioned above are not disclosed in the publications No. 10-26413, No. 10-227526, No. 11-159874 and No. 2003-207207. Therefore, these problems can not be solved even if the technology disclosed in these patent documents is used.